1. Field of the Invention
This invention relates generally to methods and apparatus for monitoring parameters associated with fluid systems, and specifically in one aspect to the non-invasive monitoring of arterial blood pressure in a living subject.
2. Description of Related Technology
The accurate, continuous, non-invasive measurement of blood pressure has long been sought by medical science. The availability of such measurement techniques would allow the caregiver to continuously monitor a subject's blood pressure accurately and in repeatable fashion without the use of invasive arterial catheters (commonly known as “A-lines”) in any number of settings including, for example, surgical operating rooms where continuous, accurate indications of true blood pressure are often essential.
Several well known techniques have heretofore been used to non-invasively monitor a subject's arterial blood pressure waveform, namely, auscultation, oscillometry, and tonometry. Both the auscultation and oscillometry techniques use a standard inflatable arm cuff that occludes the subject's peripheral (predominately brachial) artery. The auscultatory technique determines the subject's systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject's mean pressure, by measuring actual pressure changes that occur in the cuff as the cuff is deflated. Both techniques determine pressure values only intermittently, because of the need to alternately inflate and deflate the cuff, and they cannot replicate the subject's actual blood pressure waveform. Thus, continuous, beat-to-beat blood pressure monitoring cannot be achieved using these techniques.
Occlusive cuff instruments of the kind described briefly above have generally been somewhat effective in sensing long-term trends in a subject's blood pressure. However, such instruments generally have been ineffective in sensing short-term blood pressure variations, which are of critical importance in many medical applications, including surgery.
The technique of arterial tonometry is also well known in the medical arts. According to the theory of arterial tonometry, the pressure in a superficial artery with sufficient bony support, such as the radial artery, may be accurately recorded during an applanation sweep when the transmural pressure equals zero. The term “applanation” refers to the process of varying the pressure applied to the artery. An applanation sweep refers to a time period during which pressure over the artery is varied from over-compression to under-compression or vice versa. At the onset of a decreasing applanation sweep, the artery is over-compressed into a “dog bone” shape, so that pressure pulses are not recorded. At the end of the sweep, the artery is under-compressed, so that minimum amplitude pressure pulses are recorded. Within the sweep, it is assumed that an applanation occurs during which the arterial wall tension is parallel to the tonometer surface. Here, the arterial pressure is perpendicular to the surface and is the only stress detected by the tonometer sensor. At this pressure, it is assumed that the maximum peak-to-peak amplitude (the “maximum pulsatile”) pressure obtained corresponds to zero transmural pressure. Note that other measures analogous to maximum pulsatile pressure, including maximum rate of change in pressure (i.e., maximum dP/dT) can also be implemented.
One prior art device for implementing the tonometry technique includes a rigid array of miniature pressure transducers that is applied against the tissue overlying a peripheral artery, e.g., the radial artery. The transducers each directly sense the mechanical forces in the underlying subject tissue, and each is sized to cover only a fraction of the underlying artery. The array is urged against the tissue to applanate the underlying artery and thereby cause beat-to-beat pressure variations within the artery to be coupled through the tissue to at least some of the transducers. An array of different transducers is used to ensure that at least one transducer is always over the artery, regardless of array position on the subject. This type of tonometer, however, is subject to several drawbacks. First, the array of discrete transducers generally is not anatomically compatible with the continuous contours of the subject's tissue overlying the artery being sensed. This can result in inaccuracies in the resulting transducer signals. In addition, in some cases, this incompatibility can cause tissue injury and nerve damage and can restrict blood flow to distal tissue.
Other prior art techniques have sought to more accurately place a single tonometric sensor laterally above the artery, thereby more completely coupling the sensor to the pressure variations within the artery. However, such systems may place the sensor at a location where it is geometrically “centered” but not optimally positioned for signal coupling, and further typically require comparatively frequent re-calibration or repositioning due to movement of the subject during measurement.
Tonometry systems are also commonly quite sensitive to the orientation of the pressure transducer on the subject being monitored. Specifically, such systems show degradation in accuracy when the angular relationship between the transducer and the artery is varied from an “optimal” incidence angle. This is an important consideration, since no two measurements are likely to have the device placed or maintained at precisely the same angle with respect to the artery. Many of the foregoing approaches similarly suffer from not being able to maintain a constant angular relationship with the artery regardless of lateral position, due in many cases to positioning mechanisms which are not adapted to account for the anatomic features of the subject, such as curvature of the wrist surface.
Furthermore, compliance in various apparatus components (e.g., the strap and actuator assembly) and the lack of soft padding surrounding the sensor which minimizes edge effects may adversely impact the accuracy of tonometric systems to a significant extent.
One very significant limitation of prior art tonometry approaches relates to the magnitude and location of the applied applanation pressure during varying conditions of patient motion, position, mean pressure changes, respiration, etc. Specifically, even when the optimum level of arterial compression at the optimal coupling location is initially achieved, there is commonly real-world or clinical factors beyond reasonable control that can introduce significant error into the measurement process, especially over extended periods of time. For example, the subject being monitored may voluntarily or involuntarily move, thereby altering (for at least a period of time) the physical relationship between the tonometric sensor and the subject's tissue/blood vessel. Similarly, bumping or jarring of the subject or the tonometric measurement apparatus can easily occur, thereby again altering the physical relationship between the sensor and subject. The simple effect of gravity can, under certain circumstances, cause the relative positions of the sensor and subject blood vessel to alter with time as well.
Furthermore, physiologic responses of the subject (including, for example, relaxation of the walls of the blood vessel due to anesthesia or pharmacological agents) can produce the need for changes in the applanation level (and sometimes even the lateral/proximal position of the sensor) in order to maintain optimal sensor coupling. Additionally, due to the compliance of surrounding tissue and possibly measurement system, the applanation level often needs to adjust with changes in mean arterial pressure.
Several approaches have heretofore been disclosed in attempts to address the foregoing limitations. In one prior art approach, an occlusive cuff is used to provide a basis for periodic calibration; if the measured pressure changes a “significant” amount or a determined time has elapsed, then the system performs a cuff calibration to assist in resetting the applanation position. Reliable pressure data is not displayed or otherwise available during these calibration periods. See for example U.S. Pat. No. 5,261,414 to Aung, et al issued Nov. 16, 1993 and entitled “Blood-Pressure Monitor Apparatus,” assigned to Colin Corporation (hereinafter “Aung”). See also U.S. Pat. No. 6,322,516 issued Nov. 27, 2001 and entitled “Blood-Pressure Monitor Apparatus,” also assigned to Colin Corporation, wherein an occlusive cuff is used as the basis for calibration of a plurality of light sensors.
In another prior art approach, a pressure cuff or a pelotte equipped with a plethysmographic gauge, such as an impedance or a photo-electric device, is used to drive a servo control loop. See, e.g., U.S. Pat. No. 4,869,261 to Penaz issued Sep. 26, 1989 and entitled “Automatic noninvasive blood pressure monitor,” assigned to University J.E. Purkyne v Brne (hereinafter “Penaz”). In this device, the sensor is connected through at least one amplifier and a phase corrector to an electro-pressure transducer. All these components constitute the closed loop of a servo control system which (at least ostensibly) continuously changes the pressure in the cuff and attempts to maintain the volume of the artery at a value corresponding to zero tension across the arterial wall. The servo control system loop further includes a pressure vibration generator, the frequency of vibration being higher than that of the highest harmonic component of blood pressure wave. A correction circuit is also provided, the input of which is connected to the plethysmographic sensor and output of which is provided to correct the setpoint of the servo control system. The Penaz system therefore in effect constantly “servos” (within a cardiac cycle) to a fixed light signal level received from the sensor. Unlike the Colin systems described above, the system continuously displays pressure to the operator. However, the operation of the plethysmographic sensor of Penaz limited the application of this device to a peripheral section of a limb (preferably a finger) where the peripheral pressure, especially under conditions of compromised peripheral circulation, may not accurately reflect aortic or brachial artery pressure. This presents a potentially significant cause of error.
Yet another prior art approach uses a series of varying pressure “sweeps” performed successively to attempt to identify the actual intra-arterial blood pressure. The applanation pressure applied during each of these sweeps is generally varied from a level of arterial under-compression to over-compression (or vice-versa), and the system analyzes the data obtained during each sweep to identify, e.g., the largest pressure waveform amplitude. See, e.g., U.S. Pat. No. 5,797,850 to Archibald, et al issued Aug. 25, 1998 and entitled “Method and apparatus for calculating blood pressure of an artery,” assigned to Medwave, Inc. (hereinafter “Archibald”). The system of Archibald is not truly continuous, however, since the sweeps each require a finite period of time to complete and analyze. In practice the sweeps are repeated with minimal delay, one after another, throughout the operation of the device. During applanation mechanism resetting and subsequent sweep operations, the system is effectively “dead” to new data as it analyzes and displays the data obtained during a previous sweep period. This is clearly disadvantageous from the standpoint that significant portions of data are effectively lost, and the operator receives what amounts to only periodic indications of the subject's blood pressure (i.e., one new pressure beat display every 15-40 seconds).
Lastly, the techniques for non-invasive pressure measurement disclosed by the Assignee of the present invention in U.S. Pat. Nos. 6,228,034, 6,176,831, 5,964,711, and 5,848,970, each entitled “Apparatus and method for non-invasively monitoring a subject's arterial blood pressure” and incorporated herein by reference in their entirety, include modulation of applanation level at, inter alia, frequencies higher than the heart rate (e.g., sinusoidal perturbation at 25 Hz). Further, Assignee has determined over time that it is desirable in certain circumstances to control the applanation level according to other modulation schemes and/or frequencies, and/or which are not regular or deterministic in nature, such as those disclosed by co-owned U.S. Pat. No. 6,974,419, entitled “Method and apparatus for control of non-invasive parameter measurements” and incorporated herein by reference in its entirety. Each of the foregoing methods, however, distinguishes between two modes of operation, the first being (1) calibration; and the second being known as (2) patient monitoring mode (“PMM”).
“Simulated Annealing”
Simulated annealing (SA) is a term that relates to optimization schema that are related to or modeled generally after physical processes. For example, one branch of simulated annealing theory is a generalization of a Monte Carlo method for examining the equations of state and frozen states of n-body systems. The concept is based to some degree on the manner in which liquids freeze or metals recrystallize during the physical process of annealing. In an annealing process, material initially at high temperature and disordered, is cooled so as to approximately maintain thermodynamic equilibrium. As cooling proceeds, the system becomes more ordered and approaches a “frozen” ground state at Temperature (T)=0. Accordingly, SA can be thought of as analogous to an adiabatic approach to the lowest energy state. If the starting temperature of the system is too low, or the cooling regimen is insufficiently slow, the system may form defects or freeze in meta-stable states; i.e., become trapped in a local minimum energy state.
One scheme (Metropolis) selects an initial state of a thermodynamic system (energy E and temperature T), and holding T constant, the initial configuration is perturbed, and the change in energy (dE) determined. If the change in energy is negative, the new configuration is accepted. If the change in energy is positive, it is accepted with a probability determined by the Boltzmann factor exp-(dE/T). This processes is then repeated sufficient times to give adequate sampling statistics for the current temperature. The temperature is then decremented, and the entire process repeated until a “frozen” state is achieved (at T=0).
This Monte Carlo approach can be analogized to combinatorial problems. The current state of the thermodynamic system is analogous to the current solution to the problem. The energy equation for the thermodynamic system is analogous to the objective function. The ground state is analogous to the global minimum.
A significant difficulty in implementing this algorithm, however, is that there is often no obvious analogy for the temperature (T) with respect to a parameter in the combinatorial problem. Furthermore, avoidance of entrainment in local minima (quenching) is dependent on an “annealing schedule”, the choice of initial temperature, the number of iterations performed at each temperature, and how much the temperature is decremented at each step as cooling proceeds.
Based on the foregoing, there is needed an improved apparatus and methodology for accurately and continuously controlling the non-invasive measurement of parameters such as pressure. Such improved methodology and apparatus would ideally integrate the highly efficient simulated annealing (SA) approach and allow for, inter alia, continuous measurement (tonometrically or otherwise) of one or more physiologic or hemodynamic parameters, the measured values of such parameters being reflective of true (e.g., intra-arterial) parameters, while also providing robustness and repeatability under varying environmental conditions including motion artifact and other noise. In addition, such method and apparatus would operate under a substantially unified scheme, as opposed to the two or more independent schemes modeled in prior art devices.
Such a method and apparatus would also be easily utilized by trained medical personnel and untrained individuals, thereby allowing subjects to accurately and reliably conduct self-monitoring if desired.